1. Field of the Invention
The present invention relates to a connecting film, a joined structure, and a method for producing the joined structure. Specifically, the present invention relates to a connecting film capable of electrically and mechanically connecting circuit members such as IC chips and liquid crystal display panels (LCD panels) in liquid crystal displays (LCDs); a joined structure having the connecting film; and a method for producing the joined structure.
2. Description of the Related Art
Conventionally, as one means of connecting circuit members with each other, connecting material tapes (e.g., anisotropic conductive films (ACFs)) have been used, which are formed by coating a release film with a thermoplastic resin containing conductive particles dispersed therein.
The anisotropic conductive films are used for adhering and electrically connecting various terminals with each other, for example, for connecting flexible printed circuits (FPCs) and/or terminals of IC chips with indium tin oxide (ITO) electrodes on glass substrates of LCD panels.
One practically used connecting film is cationic-curable connecting films each containing a cationic curing agent and an epoxy resin. The cationic-curable connecting films realize curability at low temperatures and reduction of warpage of an adherend.
However, sulfonium salts, etc., serving as the cationic curing agent, have high curing activity and thus, the curing reaction is easily inhibited by, for example, even a trace amount of impurities, causing problematic phenomena such as poor curing.
In particular, such poor curing often occurs due to a passivation film of polyimide formed on the back surface of an IC chip. Specifically, in the case where an IC chip is connected via a cationic-curable connecting film, at the time when the cationic-curable connecting film is attached to the IC chip to initiate curing reaction, the generated cationic species (H+) are inactivated by polyimide materials of the passivation film. One possible reason why the generated cationic species (H+) are inactivated by the polyimide materials of the passivation film lies in that the cationic species (H+) react with nitrogen atoms (N) contained in polyimide and are then trapped (the reaction R3N→R3N+H occurs to form ammonium salts).
In addition, even when an IC chip is connected using a TAB tape formed by laminating a polyimide film on a Cu foil with an adhesive, the adhesive is made of polyamide and thus, causes inhibition of curing.
Another practically used connecting film is radical-curable connecting films each containing a radical curing agent (an organic peroxide) and an acrylic resin. The radical-curable connecting films are often employed for connection at the PWB side, and realize satisfactory curability at low temperatures. However, the radical-curable connecting films do not generate a hydroxyl group during curing and thus, the interaction between the film and the adherend having polarity becomes weak, causing problematic phenomena such as poor curing. In particular, the radical-curable connecting film has poor adhesion to the glass surface at the LCD panel side, problematically causing peeling at the interface.
A radical-curable connecting film in which an epoxy resin is contained is also known (Japanese Patent (JP-B) No. 3587859). In this case, the radical-curable connecting film also has poor adhesion to the glass surface.
The radical-curable connecting film, therefore, is not suitable for connection at the LCD panel side, and also is not used widely.
In addition, a rubber material commonly used as the material for a connecting film is easy to cause inhibition of curing.
Further, there is proposed a dual-curable connecting film containing a radical curing agent (for curing at low temperatures) and an imidazole curing agent (for curing at high temperatures) (for example, Japanese Application Laid-Open (JP-A) No. 2007-262412). However, a connecting film formed from components having different curing mechanisms tends to cause phase separation during curing and to involve inner cracks, resulting in degraded connection reliability. Also, curing must be performed at two steps, which is not suitable for connection in a short time.
Furthermore, proposed are a dual-curable connecting film formed from a radical curing agent and a cationic curing agent (for example, JP-A No. 2006-127776); a connecting film containing, in a binder, a heat-curable composition and a photo-curable composition (for example, JP-A No. 2005-235956); and a two-layered connecting film having a photo-cationic curing agent-containing layer and a photo-radical curing agent-containing layer (for example, International Publication No. WO00/084193). Neither of them can improve curing failure by a passivation film of polyimide formed on the back surface of an IC chip. Thus, demand has arisen for development of a connecting film which involves no curing failure by a passivation film of polyimide.
Further, in the recent display device members such as LCDs, PDPs and organic ELs, wires of metal such as Al, Mo, Cr, Ti, Cu or Ni are often formed on a base of ITO from the viewpoint of, for example, electrical conductivity. As these wires of metal lack light transparency, it is difficult to connect circuit members with each other by photo-curing a connecting film placed therebetween.
Moreover, a method is known in which difference in viscosity between the first layer and the second layer is made in a connecting film having the two layers, so as to improve efficiency of capturing conductive particles (for example, JP-A No. 06-45024). However, in this connecting film, the bonding force near the boundary between the first layer and the second layer is weak, and thus the conduction reliability may decrease.
Furthermore, a method is known in which a hydroxyl group-containing resin such as a phenoxy resin is contained in an adhesive so as to improve adhesion properties of the adhesive (For example, International Publication No. WO 00/046315). However, the adhesive disclosed in International Publication No. WO 00/046315 dose not employ two layers having mutually different viscosities, and the structure and effect of which are completely different from those of the present invention.